The invention relates to a polymer light-emitting diode (LED) which can emit near white light with a broad electoluminescence spectrum, and to methods for preparing the same. Particularly, the invention relates to a LED having an organic light-emitting layer which comprises a blue-light emitting polyparaphenylene vinylene (PPV) copolymer and an orange-red (or red) light-emitting PPV alkoxy-substituted derivatives, and a process for fibricating the same.
Since Tang et al. (Appls. Phys. Lett., 515(1987)914) has made a device having the structure ITO/diamine/Alq3/Mg:Ag wherein ITO stands for indium/tin oxide, and Alq3 for tris(8-hydroxyquinioline) aluminum, because such device had an external quantum efficience of 1% and exhibited a high brightness of 1000 cd/m.sup.2 (10V), the study of organic light-emitting diode (LED) had been rapidly promoted. At 1990, the group of Cavendish Lab of Cambridge University, England, had made an LED having structure as ITO/PPV/Ca by using PPV as light-emitting layer, ITO as anode, and Ca as cathode, which had a quantum efficiency of 0.05% (Nature, 347(1990)539; U.S. Pat. No. 5,247,190 (1993); U.S. Pat. No. 5,425,125(1995); U.S. Pat. No. 5,401,827(1995)).
Heretofore, the most primary organic LED device comprises a single layer organic light-emitting layer which is interposed between a transparent electrode as anode) and a metal electrode (as cathode). Additionaly, in order to enhance the emission efficiency of the organic LED device, it can have two organic layers, the first layer as hole transport layer and the second layer as organic light-emitting layer, or, the first layer as organic light-emitting layer and the second layer as election transport layer. These two organic layers are interdisposed between a transparent electrode (as anode) and a metal electrode (as cathode). Furthermore, there are devices having three organic layers in an order as the hole transport layer, the organic light-emitting layer and the elentron transport layer, which are interdisposed between a transparent electrode (as an anode) and a metal electrode (as a cathode). The light-emitting process of this device comprises of, after applying a bias on such an LED, moving of holes and electrons from the anode and the cathode, respectively, under driving of electric field, transisting over their respective energy barriers and encountering at the light-emitting layer to form excitons which decay from the excited state to ground state and emit light.
PPV-based polymer has been extensively used in the fibracation of LED due to their excellent fluorescent property, however, since PPV is insoluble and in fusible, it has to coat by using a solution of precursor thereof and carry out an eliminating reaction by heating under vacuum to obtain PPV. In order to simplify the process for fibricating such devices, long chain alkyl or alkoxy had been attached onto the side chain of PPV (U.S. Pat. No. 5,408,109(1995)) at the aim of increasing the solubility thereof such that they can dissolve in common organic solvents, and at the same time, change their energy gaps. Alternatively, a block copolymer had been synthesized firstly by Karasz (Macromolecules, 26(1993)1188, Macromolecules, 26(1993)6570) through Wittig reaction, which comprised a rigid segment and a soft segment, wherein the rigid segment was the paraphenylene vinylene segment which could change the color of light by varying length thereof, and wherein the soft segment included the alkyl, ether groups and ester groups, which functioned in enhancing the solubility and film-forming property.
Currrently, light colors emitted by polymer LED include blue, green and even to infrared, which can be determined through selecting from a single material or through blending of two or more of polymers. The earliest study in this field had been conducted by Yoshino (Jpn. J. Appl. Phys., 32(1993)L921) who blended poly(di-octyloxy phenylene vinylene (PdPOV) with Alq3 and then fibricated an device whose light color could change gradually from orange (light color of PdOPV) to yellowish-green (light color of Alq3) with increase of applied voltage. Heeger (J. Elec. Mater, 20(1991)945) blended poly(2-cholestanoxy-5-thexyldimethylsiltyl-1,4-phenylene vinylene)(CS-PPV) with 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole)(PBD) and then fibricated a device, and found that amount of PBD had not any effect on the shape of electroluminescent (EL) spectrum. Further, Karasz (J. Appl. Phys., 76(1994)2419) blended poly (9-vinylcarbazole) (PVK) with conjugated non-conjugated multiblock PPV copolymer (CNMBC) in various ratios and then fabricated an device therefrom, and found that, at PVK content of 3%, a new peak appeared in the EL spectrum thereof which was different to spectra of original two polymers, due to formation of an exciplex from the interaction between PVK and CNMBC.
Inganas (Nature; 372(1994)444) used blends of polythiophene derivatiaves having different substitutent as light-emitting layers and found that, under an applied voltage, it could emit initially a light color generated from polymer having a low energy gap and as the voltage was increased, light color generated from polymer having a high energy gap began to appear, and thus obtained a device whose light color could be controled by a voltage. In 1996, this research group (Appl. Phys. Lett., 68(1996)147) blended three polythiophene derivatives, that is, poly(3-methyl-4-octylthiophene) (PMOT), poly(3-cycolhexylthiophene) (PCHT), and poly [3-(4-octylphenyl)-2,2'-bithiophene](PTOPT), with polymethyl methacrylate (PMMA) as binder, and found an optimal white light emitting conditions at weight ratio of PMOP:PCHP:PTOPT:PMMA as 10:4:1:1, wherein, under a operating voltage of 20 V, .lambda.max of EL spectrum thereof were 465 nm and 620 nm. However, under a operating voltage between turn-on voltage and 20 V, its light color changed with applied voltages.
In summary, heretofore, electroluminescent spectrum of organic LED can extend from ultraviolet to infrared. Emission spectra of LED may be affected by conjugated structures in polymer main chains, and additionally, energy gaps thereof can be controlled by incorporating substitutends having different functionalities so that a variety of light color can output, Furthermore, by blending several kinds of polymer each having different energy gap and controlling various applied voltages, a variety of light color can be output and even white light-emitting diodes can be obtained. However, white light-emitting diodes cited in literatures emit red light at low voltage and could emit white light only at 20V. In this respect, after an intensive study, the inventor of this application obtained eventualy a novel polymer LED which can emit near white light with a broad electroluminescent spectrum under various voltages.